1. Field of the Invention
This invention relates to a head feed device to be employed in a magnetic disc recording apparatus and, more particularly, in an electronic camera of the kind arranged to record still pictures on a compact magnetic disc.
2. Description of the Related Art
FIGS. 1(A) and 1(B) of the accompanying drawings show, by way of example, the magnetic head feed device of the conventional apparatus of the above-stated kind. Referring to FIGS. 1(A) and 1(B), a motor 3 is secured to a chassis 1. A spindle 4 is mounted on the rotating shaft of the motor. A core part of a magnetic disc 2 is arranged to be secured to the spindle 4. A carriage 6 is slidably mounted, via oil impregnated sintered bearings 8 and 8', on two guide shafts 7 and 9 which are secured to the chassis 1.
On the carriage 6 is mounted a magnetic head 5 which is arranged to be used solely for recording or solely for reproduction or both for recording and reproduction. A feeding helical screw shaft 11 is arranged in parallel with the guide shaft 9. The two ends of the screw shaft 11 are rotatably carried through bearings 10 and 10'. To one end of the screw shaft 11 is secured a gear 12. The gear 12 engages a gear 13 which is secured to the output shaft of a stepping motor 14. The carriage 6 is provided with a projection 6a which protrudes from the base part of the carriage 6. The projection 6a engages the above-stated screw shaft 11. A datum face 6b is formed on the reverse side of the fore end part of the carriage 6. The datum face 6b is in contact with the upper side of the guide shaft 7. The position of the magnetic head 5, in the direction perpendicular to the face of the magnetic head 5 confronting the surface of the magnetic disc 2, is defined jointly by the datum face 6b and the guide shaft 7.
The electronic still picture camera is generally standardized to form 50 circular recording tracks on a compact magnetic disc 2 at a pitch of 100 .mu.m. According to the standards generally accepted, the positional precision of the recording tracks must be set at .+-.14 .mu.m. Positional precision required for the magnetic disc 2 becomes severer in the light of expansion and contraction of the disc 2 due to temperature and humidity. The most important factors upon which the positional precision of the magnetic head 5 hinges, include the thread cutting precision of the feed screw shaft 11 and the rotating angular precision of the stepping motor 14.
However, even if the feed screw precision is adequately obtainable, the positional precision of tracks is hardly obtainable if the degree of parallelism between the screw shaft 11 and the guide shaft 9 is insufficient. The reason for this is as follows:
Referring now to FIG. 2, let us assume that the magnetic head is shifted to an extent (5 mm) corresponding to 50 recording tracks by the rotation of the shaft 11. Then, in case that the screw shaft 11 is perfectly in parallel with the guide shaft 9 as indicated by a full line in FIG. 2, an error in the position of tracks arises as an error in the pitch of the feed screw. Assuming that the sectional shape of the thread groove of the screw shaft 11 is in a pedestal shape of an inclination of 30 degrees, if the contact point of the projection 6a with the screw shaft 11 is deviating d .mu.m from the axis of the screw shaft 11 in the radial direction, the deviation of the head shifting direction in the radial direction becomes d tan 15 degrees. The degree of deviation d is dependent upon the degree of parallelism between the screw shaft 11 and the guide shaft 9. An error in the parallelism results in the track position error as indicated by a broken line in FIG. 2. Assuming that the thread cutting precision is .+-.5 .mu.m or thereabout, if the track position error as a whole is to be kept within .+-.10 .mu.m, a condition for that degree of precision becomes 10-5&gt;d tan 15 degrees. Then, the extent of deviation due to the error in the parallelism between the screw shaft 11 and the guide shaft 9 must be kept to a value not exceeding 18 .mu.m.
However, the structural arrangement of the conventional head feed device does not assure the above-stated degree of parallelism because the bearing 10 of the screw shaft 11 and the guide shaft 9 are separately mounted on the chassis as shown in FIG. 1(A).